It has now generally been recognized that while heat generated during daylight hours can be stored by various means and used when the heat of the sun is insufficient to maintain a desired temperature and heat energy must be provided by other means, such as the burning of fossil fuels, there is also a definite need for storing what might be considered cooling capacity. By the use of that term I refer to heat energy stored at temperatures substantially lower than room temperature, i.e., 70.degree. F. Thus, the present invention is most specifically directed to the storage of cooling capacity, and particularly such storage due to a change of phase of the storage composition from liquid to solid.
Perhaps the most common forms of cooling capacity storage on a commercial scale are the utilization of chilled water storage or ice storage. Chilled water storage is accomplished by chilling water at off-peak hours, for example, during the night and early morning when the consumption of electricity is less and electricity rates are therefore lower, and then utilizing that chilled water to air condition a building during peak periods when time-of-use electricity rates are higher.
Chilled water storage, however, presents some important problems, although it has achieved a measure of commercial utility. Among those problems is that a great deal of space is required to be occupied by the chilled water, which is generally stored in tanks. Since the chilled water maintains its cooling capacity solely by the specific heat of the water, i.e., there is no change of phase during which coolness capacity can be stored, large volumes of water are necessary in order to effect any substantial chilling capacity. Large volumes of chilled water require storage tanks with large volumes. Moreover, accommodating the return flow of chilled water the coolness capacity of which has been utilized in order to air condition a building, presents a still additional storage requirement. After its cooling capacity has been so used, the chilled water, now at a substantially higher temperature, is returned, presumably to the tank in which it was originally stored. In this case some means must be provided to thermally separate the returning, warmed water from chilled water still in the tank, or the warmed water must be separately stored in a tank that is not in heat transfer relation with the original chilled water storage tank. It has thus become recognized that while chilled water storage may not be exceptionally difficult to accomplish, it has infirmities of space that severely limit its economic viability.
In order to overcome the weaknesses of chilled water storage, ice storage has long been considered a possible viable alternative. Thus, air conditioning of a building, such as a church, which is used for relatively short periods of time, by means of ice is not a new practice. Yet, while the excessive storage volume problems that are attendant with chilled water storage are not present with ice storage, the latter presents problems of its own. For example, ice storage systems, which may be of various constructions, are generally established above grade and occupy space that is valuable for other applications. In chilled water storage the tanks for the chilled water may be located below grade, so that they will underlie facilities at ground level, e.g., a parking lot or lawn. Ice storage units are often located above grade and so occupy valuable space usable for other purposes, besides being visually unattractive. In addition, freezing water at 32.degree. F. requires low-temperature refrigeration equipment. It will be apparent that more compressor energy is necessary to freeze water at 32.degree. F. than to freeze materials at higher temperatures and that the energy required exceeds that on a straight-line basis. There are other problems with ice storage, such as the fact that water expands as it freezes, thereby subjecting to undesirable stresses any containers for water to be frozen.
That type of coolness storage system which appears to hold greatest promise for commercial utilization is one which is of more recent advent and is now effecting penetration into the marketplace. That type of system is generally referred to as one making use of phase change materials, which have a freezing and melting point above 32.degree. F. and which do not have a substantial change of density when they transmute between solid and liquid phases. In this manner such phase change materials, or PCM's, can be frozen to a solid during off-peak hours, then utilized for their heat of fusion, as well as specific heat, during peak hours. Among such PCM's are those based on Glauber's salt, modified by the addition of other salts in order to obtain the requisite freezing point.
In my U.S. Pat. No. 4,689,164, issued Aug. 25, 1987 and assigned commonly herewith, I have disclosed an inproved phase change material based on Glauber's salt, i.e., sodium sulfate decahydrate, in which there is a ratio of potassium chloride and ammonium chloride such that the resulting salt has a eutectoid point, or freezing-melting point, of about 47.degree.-48.degree. F. While such sodium sulfate-based PCM has achieved a place in the market, in practicality I have found it impossible, at least up to this time, to devise a Glauber's salt-based PCM that has a freezing-melting point plateau of less than 47.degree. F. Yet such a lower freezing point would appear highly advantageous, since chillers used to produce cold water for bringing the temperature of PCM's below their freezing point have now progressed to a degree where they can easily produce water in a 35.degree.-38.degree. F. range or lower. As a consequence, it is presently highly desirable to find a PCM that has a freezing point below 47.degree. F., because colder water is sometimes required for dehumidification.
While phase change materials based on Glauber's salt occupy perhaps one-fourth to one-third of the volume required for chilled water storage, and while they do not have the problem of segregating the return flow of warmed water after it has been utilized as a coolant for air conditioning a building, PCM's do have an economic disadvantage when compared with water, for example. While sodium sulfate-based phase change materials as disclosed in my Pat. No. 4,689,169 tend to mitigate those difficulties, cost does play an important role and Glauber's salt-based PCM's are not inexpensive. Considering that a three-year payback is at times almost an unwritten rule of whether a PCM-based coolness storage system will be utilized in a new building, the PCM art is dependent on cost cutting improvements for its economic survival, and it is necessary to determine further means by which the cost of such PCM's can be reduced.
As stated above, I am aware of no sodium sulfate-based PCM that can be adjusted to a freezing point substantially less than 47.degree. F. This is not only a problem in regard to such lower temperatures, but one that is endemic to the use of PCM's in general. PCM's do not appear to freeze and melt at a point that can be easily adjusted. Thus, in may patented composition, the PCM there disclosed freezes and melts at about 47.degree. F., perhaps over a range or plateau from 46.5.degree. to 48.degree. F. Yet changes in materials utilized do not permit the adjustment of the freezing-melting point to 45.degree. F. or to 50.degree. F. Apparently any specific combination of salts form a phase diagram that has one or more peritectic (or eutectoid) points, and the Glauber's salt phase diagram with potassium chloride and ammonium chloride has a eutectoid point at 47.degree. F. A change in amounts of ingredients that might make it appear that such composition would freeze, e.g., at 50.degree. F. will not only not freeze at that temperature, but will form a slush that will not freeze solid over a wide range of temperature. Also, such PCM's may melt incongruently and require shallow containers. Thus, it is particularly desirable to formulate other PCM's that will freeze at other desirable temperatures.
As a consequence, it is a principal object of the present invention to provide a phase change material that will freeze and melt at a temperature below 47.degree. F. but above 32.degree. F., and most preferably at approximately 40.degree. F. to 42.degree. F.
Another primary object is to formulate an inorganic composition for use as a phase change material, which composition will be less expensive per unit of coolness stored than the salts that make up a Glauber's salt-based PCM for coolness storage, and which will have a relatively high heat of fusion when compared to Glauber's salt-based compositions. Such inorganic compositions should be in plentiful supply and have a high product density as well.
While the ideal formulation has yet to be discovered, it is believed that the invention disclosed herein represents a major effort in reaching these objectives and that, particularly where I disclose the use of an inorganic composition that melts and freezes at approximately 41.degree. F., this is an important complement sodium sulfate-based PCM's that freeze at 47.degree. F. and represents a pioneer step forward in the art.